In gas turbine engines there is no independent supply or control of airflow to the combustor. Firing the combustor drives the turbine which is direct connected to the compressor, thereby supplying the airflow.
During load operation fuel is input to maintain a controlled power output, speed and/or temperature The fuel air ratio is dictated by the gas turbine design, varying somewhat through the load range and during transients. An increase in fuel flow increases gas temperature, initiating a turbine speed increase. This increases the compressor speed providing an increased airflow. The rate of change of fuel flow is limited to, among other things, avoid excessive swings in fuel air ratio.
In the event of a flameout in a combustor, fuel continues to be introduced by the control system in an attempt to meet demand. Airflow continues as the unit coasts down. This introduction of unburnt fuel into the air produces an explosive mixture which could be set off by any hot spot or spark. The potential magnitude of any explosion is a function of the amount of fuel present and the stoichiometry of the mixture.
Flame scanners in gas turbine engine combustors are not desirable because of sooting problems and background incandescence of materials inside the combustor and turbine area. A simple reliable means for detecting a flameout and effecting a timely trip of the fuel is desired.